The electronic industry requires the continuous supply of ultra-pure nitrogen, having typically a purity of the order of 1 to 100 ppb (parts per billion or 10.sup.-9) of impurity.
By the usual method, there is produced directly ultra-pure nitrogen in an air distillation installation called "HPN (High Purity Nitrogen) of electronic type". This comprises a nitrogen production column in which compressed air, purified with heating of hydrogen and carbon monoxide by catalysis and then purified by adsorption of water and carbon dioxide, is cooled to about its dew point, then introduced into the base of a distillation column having a number of theoretical plates sufficient to produce ultra-pure nitrogen at the head. The column is provided with a head condenser cooled by "rich liquid" (air enriched in oxygen) collected in a sump and expanded.
The installation is provided with liquefaction means of a portion of the ultra-pure nitrogen, and this liquid nitrogen is sent to standby storage. In the case of misfunction of the installation or of a loss of electric power, this storage permits ensuring the continuity of supply of ultra-pure nitrogen.
This technique has serious drawbacks:
if the outage is prolonged, the storage will be exhausted. There is accordingly needed a very large volume storage, which is quite costly: 200,000 liters to supply 1,000 Nm.sup.3 /h for four days, which is not necessarily the maximum length of the outages in question;
no suitable solution exists if the storage is emptied, because one cannot have recourse to ordinary liquid nitrogen or nitrogen of medium purity, which is to say having a medium purity of the order of ppm (parts per million or 10.sup.-6);
when the storage is emptied following a breakdown of the installation, it is necessary once more to refill it, which takes quite a bit of time. For example, if the installation produces 7% of ultra-pure nitrogen in liquid form, two months are required to restore the reserve (200,000 liters) after four days of shutdown of the installation. During this period, the whole system is vulnerable.